1. Field of the Invention
The present invention relates to an electrophotographic image-forming apparatus.
2. Description of the Related Art
A conventional electrophotographic image-forming apparatus performs an electrophotographic process. A charging unit (e.g., charging roller) charges the surface of an image bearing body (e.g., photoconductive drum). Then, a developing section (e.g., developing roller) applies a thin layer of toner to the electrostatic latent image electrostatically, thereby developing the electrostatic latent image into a toner image. A transfer section transfers the toner image onto a print medium. A cleaning section removes residual toner that failed to be transferred to the print medium.
One such conventional image-forming apparatus is disclosed in Japanese Patent Laid Open No. 11-184190. This image-forming apparatus performs correction of toner density of printed images in addition to the aforementioned electrophotographic image formation. The correction of toner density of printed images is to maintain the toner density that would otherwise change over time or due to changes in environmental conditions. In the correction of toner density, a test pattern is formed on an image bearing body or an intermediate transfer belt. A toner density detecting section detects the toner density of the test pattern. The toner density varies in accordance with various print conditions such as the amount of light emitted from an exposing unit, a developing voltage applied to a developing roller, and a toner supplying voltage applied to a toner-supplying roller. Thus, these print conditions are controlled based on the toner density detected by the toner density detecting section.
For example, the ability of toner to acquire charge varies due to changes in environmental conditions. A developing blade and the toner-supplying roller wear over time so that the density of toner increases due to increases in the thickness of toner layer. In order to prevent the toner density from increasing, the developing voltage, for example, is decreased below a predetermined level to decrease the toner density. Too low a developing voltage causes graininess (i.e., reproducibility of dots) to deteriorate. Thus, it is necessary not to decrease the developing voltage below a certain lower limit.
When the toner density decreases, the developing voltage is controlled in such a way that the toner density increases. Conversely, too high a developing voltage causes soiling of images or gives rise to “after-images”. Thus, it is necessary no to increase the developing voltage above a certain upper limit.
The aforementioned conventional image-forming apparatus suffers from the following drawbacks. The developing voltage is controlled within a range having a fixed upper limit and a fixed lower limit. Therefore, the graininess can deteriorate or soiling of images may occur depending on changes in the conditions of the apparatus over time and/or changes in environmental conditions.